Existing hydraulic platform lifts for trucks typically consist of a pair of vertically-aligned hydraulic cylinders attached at their respective lower ends to a platform. Each hydraulic cylinder comprises a piston member, which is attached (usually via a shaft or piston rod member) to one end of the platform which is to be raised and lowered. It is intended in most cases (with perhaps the exception of U.S. Pat. No. 5,791,860 to a self-leveling platform) that the hydraulic cylinders move in unison, so as to evenly raise and lower the platform. In certain cases, hydraulic lifts having hydraulic circuits of a specific configuration frequently experience unintended non-uniform movement of piston members within the hydraulic cylinders relative to each other due to uneven forces on each of the two cylinders due to uneven distribution of load on the platform. This results in tilting of the platform when being raised or lowered thereby constituting a safety hazard should the load on the platform be caused to shift and/or fall from the platform.
The problem of non-even movement of piston members within the hydraulic cylinders has been significantly reduced in the prior art by certain configurations for the hydraulic circuitry for such platform lifts. By way of example, U.S. Pat. No. 5,110,251, while it discloses a further embodiment of providing a particular relationship between the cross-sectional size of the first cylinder at the rod end to the cross-sectional area of the second cylinder at the cap end, incidentally discloses a particular hydraulic circuit for a hydraulic circuit for a truck liftgate, whereby as is illustrated in FIG. 2 (designated “Prior Art”), in order to raise the platform 21 hydraulic fluid is supplied from a hydraulic pump 11 to a first “double-acting” hydraulic cylinder 25 at the rod end 40 thereof. Upon the piston member 22 thereof being forced upward as a result, an equal amount of hydraulic fluid is forced from the cap end 41 thereof to the rod end 40 of the second cylinder 27, thereby causing the piston member 22 therein to rise an equal amount in the second cylinder 27. Advantageously, because the volume of hydraulic fluid forced into the first cylinder 25 at the rod end 40 is equal to the volume of hydraulic fluid thereby forced out from the cap end 41 of the first cylinder 25 and into the rod end 40 of the second cylinder 27, the movement of the piston members 22,23 in each of the cylinders 25,27 is caused to be substantially identical, regardless of the different forces which may be applied to the respective cylinders 25,27 due to a variation or “off-centering” of a load “L” which may be placed on the platform. The hydraulic circuit of U.S. Pat. No. 5,110,251, by teaching a solenoid-operated flow reverser valve 28 as shown in FIG. 2 (“Prior Art”) attached hereto, is able to provide the platform 21 with not only “power up” but also “power down” capability, with in each case the benefit of substantially uniform displacement of the piston members 22 within the respective hydraulic cylinders 25,27. As is more particularly described in detail below, the hydraulic lift and hydraulic circuit of the present invention is able to achieve the benefits and advantages of the hydraulic lift disclosed in U.S. Pat. No. 5,110,251, but in a substantially different manner and in a way which produces at least one clear advantage over U.S. Pat. No. 5,110,251.
U.S. Pat. No. 5,791,860, also to the same inventor as the invention herein, teaches a hydraulic platform lift for a truck or truck trailer, and is directed towards a hydraulic platform having hydraulic and electrical circuitry to provide automated, or automated with manual override, platform leveling capability in the event the truck is tilted. U.S. Pat. No. 5,791,860 discloses three embodiments where leveling may be accomplished in three separate ways; a first embodiment where leveling is accomplished by stopping movement of one hydraulic cylinder and allowing the other hydraulic cylinder to “catch up”, a second embodiment where leveling is accomplished by reversing the direction of movement of one hydraulic cylinder relative to the other to allow rapid leveling, and a third embodiment where the rate of movement of one piston member in a hydraulic cylinder is slowed relative to the piston member of the second hydraulic cylinder, to allow the piston member in such second hydraulic cylinder to “catch up”. Incidentally, however, U.S. Pat. No. 5,791,860 teaches in inter alia FIGS. 3B, 4B, 5B, 6B and as more simplistically set out in FIG. 3 (“Prior Art”) hereto, a hydraulic lift platform 21 for a truck wherein each hydraulic cylinder 25,27 is supplied with hydraulic fluid from a separate pump 11. A belt, gear, or shaft member 30 is provided connecting the two identical pumps 11, in order that the pumps 11 run at precisely the same speed and accordingly each provide precisely the same volume of hydraulic fluid to each of the respective first and second hydraulic cylinders 25,27. Accordingly, each of the piston members 22 within each of corresponding hydraulic cylinders 25,27, due to being provided with identical volumes of hydraulic fluid per given time unit, thereby move at equal rates and in unison regardless of any unequal forces which may be applied due to uneven load distribution on the platform 21. In order to provide “power up” and “power down” capability, solenoid-activated flow reverser valves 90 are provided in each hydraulic circuit for each hydraulic cylinder 25,27. Accordingly, like the hydraulic configuration of U.S. Pat. No. 5,110,251, the hydraulic lift circuit of FIG. 3 (“Prior Art”) provides for uniform movement of piston members 22 regardless of unequal forces which may be applied due to uneven load distribution on the platform 21. Again, however, as is more particularly described in detail below, the hydraulic lift and hydraulic circuit of the present invention is able to achieve the benefits and advantages of the hydraulic lift disclosed in U.S. Pat. No. 5,791,860 but in a substantially different manner and in a way which produces at least one clear advantage over U.S. Pat. No. 5,791,860.